For ease of understanding the invention will be described in terms of a primary intended application, namely hot work steels and, for further convenience, closed die drop forging. In drop forging a die is formed having a cavity which contains the desired shape of the part to be formed in negative. A metal blank from which a finished part will be fabricated is heated to a plastic condition before the blank is placed in the cavity. The upper and lower dies which form the die set are operated by powerful hammers which plastically deform the heated metal blank into a finished part which conforms to the shape of the die cavity. To be economically feasible, a die must be capable of forging thousands of parts before the die needs to be resunk.
From the foregoing description it will be appreciated that the steel from which closed die drop forging die sets are made is subjected to extremely rugged operating conditions. The die steel must be sufficiently hard so that wear of the dies, that is, expansion of the die cavity to a degree that the cavity goes oversize and produces an out of specification final product, is minimized. At the same time the die steel should have a uniform hardness throughout its depth so that the production obtained from the first and succeeding resinkings is substantially equal to the production obtained from the initial sinking. Stated another way, the die should have good through hardenability, that is, a constant or near constant hardness at all depths beneath the surface. This is in contrast to some steels in which the center portions, of large blocks particularly, tend to have a lower hardness than those portions near the surface. As those skilled in the art appreciate the property of having a uniform hardness from the surface to the center of a piece is generally referred to in the trade as hardenability, not to be confused with hardness.
In order to remain competitive with increased competition from alternative methods of forming parts, today's die block maker must offer die blocks that provide lengthened life. For example, a standard high quality die block having a nominal composition of C-.55, Mn-.85, Ni-.95, Cr-1.00, Mo-.38 and V-.05 has produced 50,000-60,000 parts on such products as pliers or other hand tools on the first and succeeding sinkings of the cavity. However, from the point of view of a manufacturer of hot work implements, there is still room for improvement over steels currently used because dies lasting 50,000-60,000 forgings between sinkings may not be competitive with casting methods, or may not be economical enough on a per piece basis to inhibit manufacturers from designing around or away from forged parts.
It has been discovered that the use of nitrogen as an alloy for low alloy die steels when present in controlled amounts, and in balance with certain other elements which affect the final N content of the steel, particularly manganese, improves both hardenability and wear resistance.
Nitrogen has previously been used as an alloy in stainless steels and high chromium steels. Furthermore, it is known that nitrogen is quite soluble in steels containing high amounts of chromium. Nitrogen has also been used in the past when conventional alloys have been unobtainable as hardening agents in special applications. However, it is believed that nitrogen has not previously been used as an alloy, and particularly as a contributor to hardenability, in low alloy steels.
The primary reason why nitrogen has not been used as an alloying element in low alloy steels is the low solubility of nitrogen in low alloy steels. Specifically, the solubility of nitrogen depends on the steel chemistry. As noted above, nitrogen is soluble in high chromium type steels and is also soluble in high manganese containing steels, but low alloy steels generally contain less than about 5% Cr and less than about 1% Mn.
In the present invention, nitrogen is held in solution in low alloy steels using titanium and aluminum in amounts sufficient to form titanium carbo-nitride and aluminum nitride without concurrent formation of titanium oxides and aluminum oxides, commonly known as dirt. The net result is low alloy steel with a substantial nitrogen content that is characterized by high hardenability, excellent hardness and good wear resistance.